Switch mode power supplies or switching regulators, also referred to as DC to DC converters, are often used to convert an input supply voltage to a desired output voltage at a voltage level appropriate for the internal circuitry of an integrated circuit. For example, a 5 volts supply voltage provided to an integrated circuit may need to be reduced to 2.8 volts on the IC chip to operate the internal circuitry on the chip. A switching regulator provides power supply function through low loss components such as capacitors, inductors, and transformers, and power switches that are turned on and off to transfer energy from the input to the output in discrete packets. A feedback control circuit is used to regulate the energy transfer to maintain a constant output voltage within the desired load limits of the circuit.
A switching regulator can be configured to step up the input voltage or step down the input voltage or both. Specifically, a buck switching regulator, also called a “buck converter,” steps down the input voltage while a boost switching regulator, also called a “boost converter,” steps up the input voltage. A buck-boost switching regulator, or buck-boost converter, provides both step-up and step-down functions.
The operation of the conventional buck switching regulator is well known and is generalized as follows. A conventional buck switching regulator includes a pair of power switches which are turned on and off to regulate an output voltage to be proportional to a reference voltage. More specifically, the power switches are alternately turned on and off to generate a switching output voltage at a switching output node, also referred to as the switch node. The switch node is coupled to an LC filter circuit including an output inductor and an output capacitor to generate an output voltage having substantially constant magnitude. The output voltage can then be used to drive a load.
In particular, the pair of power switches is often referred to as including a “high-side power switch” and a “low-side power switch.” The high-side power switch is turned on to apply energy to the output inductor of the output filter circuit to allow the current through the inductor to build up. When the high-side power switch is turned off, the voltage across the inductor reverses and the current through the inductor reduces during this period. As a result, the inductor current ripples above and below the nominal output current. A relatively constant output voltage is maintained by the output capacitor. The low-side power switch is turned on and off for synchronous control operation.
FIG. 1 is a schematic diagram of a conventional buck switching regulator. Referring to FIG. 1, a switching regulator 1 includes a pair of power switches S1 and S2 configured to receive an input voltage VIN and are alternately turned on and off to generate a switching output voltage VSW at a switch node (SW) 22. The switching output voltage VSW is directly coupled to an LC filter circuit including an output inductor L1 and an output capacitor COUT to generate a regulated output voltage VOUT at a node 26 having a substantially constant magnitude. The output voltage VOUT can then be used to drive a load 30 whereby switching regulator 1 provides the load current ILOAD to maintain the output voltage VOUT at a constant level.
Switching regulator 1 includes a feedback control circuit to regulate the energy transfer to the LC filter circuit to maintain the constant output voltage within the desired load limits of the circuit. More specifically, the feedback control circuit causes power switches S1 and S2 to turn on and off to regulate the output voltage VOUT to be proportional to a reference voltage VREF or to a voltage value related to the reference voltage VREF. In the present embodiment, a voltage divider including resistors R1 and R2 is used to divide down the output voltage VOUT which is then fed back to the switching regulator 1 as a feedback voltage VFB on a feedback node 28. The feedback voltage VFB is compared with the reference voltage VREF at an error comparator 12. The comparator output is coupled to a controller and gate drive circuit 14 to generate control voltages for the power switches based on a switching regulator control scheme. The control voltages are used to generate gate drive signals for the power switches S1 and S2. The gate drive signal for the high-side power switch S1 is coupled to a high-side driver circuit 18 while the gate drive signal for the low-side power switch S2 is coupled to a low-side driver circuit 20. Driver circuits 18, 20 convert the respective gate drive signals to gate drive voltages appropriate for turning on and off the respective power switches.
Buck switching regulators or “buck regulators” with fixed on-time control are preferred in the industry for some important advantages as good efficiency for light load in PFM (pulse frequency modulation) mode, easy synchronization with external signals, easy control of a relatively large off-time and a very small fixed on-time to regulate a high input voltage to a low output voltage. Fixed on-time (or constant on-time) regulators are one type of voltage regulators employing ripple-mode control where the output voltage is regulated based on the ripple component in the output signal. Because of the switching action at the power switches, all switch-mode regulators generate an output ripple current through the switched output inductor. This current ripple manifests itself as an output voltage ripple due, principally, to the equivalent series resistance (ESR) in the output capacitors placed in parallel with the load. The ESR of the output capacitor COUT is denoted as a resistor RESR in FIG. 1. Recently, low ESR capacitors are preferred to realize improved efficiency in switching regulators but the low ESR capacitors do not generate enough output ripple for feedback control. In that case, a ripple injection circuit (not shown in FIG. 1) is used to introduce the ripple signal used in the feedback loop. U.S. Pat. Nos. 7,482,791 and 7,482,793 illustrate examples of ripple injection circuits that can be applied in buck regulators using fixed on-time control.
FIG. 2 is a voltage waveform illustrating the output voltage ripple on the feedback voltage VFB of a constant on-time voltage regulator. In operation, a constant on-time (or fixed on-time) regulator switches the output inductor high for a fixed on-time (Ton) when the output ripple falls below a single reference point VREF. At the end of the fixed on-time, even if the output ripple may still be below the single reference point, the output inductor is switched low for a minimum off-time before getting switched back high for the fixed on-time again. In the feedback control loop, the output voltage ripple on the feedback voltage VFB is regulated so that the valley of the voltage ripple essentially sits at the reference voltage level (VREF), as shown in FIG. 2. The voltage ripple at the feedback node 28 increases for the fixed on-time (Ton) when the high-side power switch is turned on and the voltage ripple at feedback node 28 decreases when the high-side power switch is turned off, and the low side switch is turned on, until the feedback voltage VFB reaches the reference voltage VREF.
For voltage regulators using ripple-mode control, while the output ripple is useful in output voltage regulation, it is undesirable because the output ripple introduces an offset to the output voltage, reducing the output signal accuracy. In particular, the average DC voltage (the mid-point) of the ripple voltage signal should be equal to the comparator reference voltage VREF. However, as shown in FIG. 2, when the feedback voltage includes a ripple voltage component, the average DC voltage (line 36) of the feedback signal VFB is offset from the reference voltage VREF (line 34) since the comparator, which is referenced to the reference voltage VREF, is initiating an on-time when the ripple falls below VREF. Hence, it is the lower apex of the triangular waveform that is being regulated, not the average of the triangular waveform. As a result, output voltage VOUT has a DC offset voltage component which affects the accuracy of the buck regulator. For example, the offset at the feedback voltage may be about 10 mV but the offset is gained up through the control circuit and the power switches, due to the presence of the resistor divider (R1/R2), so that the output voltage may experience an offset voltage of 30-50 mV which is unacceptable in most applications. Also, the ripple amplitude will be dependent on load current and the converter input voltage, thus degrading load regulation and PSRR performance.
Furthermore, voltage regulators using ripple-mode control often suffers from instability when the output capacitor has very low ESR. The instability results from the feedback signal VFB not being in phase with the current flowing in the output inductor. The instability adversely affects the transient response of the ripple-mode control voltage regulators.